Wet strength resins are often added to paper products including paperboard at the time of manufacture. In the absence of wet strength resins, paper normally retains only 3% to 5% of its strength after being wetted with water. However, paper made with wet strength resin generally retains at least 10%-50% of its strength when wet. Wet strength is useful in a wide variety of paper applications, some examples of which are toweling, milk and juice cartons, paper bags, and liner board for corrugated containers.
As stated in Handbook for Pulp and Paper Technologists, Gary A. Smook, Angus Wilde Publications, 1992 (which is incorporated herein by reference): “Paper has traditionally been defined as a felted sheet formed on a fine screen from a water suspension of fibers. Current paper products generally conform to this definition except that most products also contain non-fibrous additives. Dry forming methods are now utilized for the manufacture of a few specialty paper products. Pulp is the fibrous raw material for papermaking. Pulp fibers are usually of vegetable origin, but animal, mineral, or synthetic fibers may be used for special applications. The distinction between paper and paperboard is based on product thickness. Nominally, all sheets above 0.3 mm thickness are classed as paperboard; but enough exceptions are applied to make the distinction somewhat hazy.”
Because of increased commercial emphasis on developing paper products based on recovered or recycled cellulose, there is growing interest in developing paper which is readily repulpable. Paper and paperboard waste materials are difficult to repulp in aqueous systems without special chemical treatment when they contain wet strength resins.
Improving the repulpability of paper containing wet strength resins has generally been achieved by modifying the repulping conditions. However, many conventional repulping processes used for wet strength paper result in the formation of environmentally undesirable chlorine-containing degradation products, involve strong oxidizing agents, or proceed slowly.
There is a need for improved methods for making paper products that will be readily repulpable without significantly lowering the wet and dry strength properties of the paper.